This specification relates to a package for a MEMS device. The disclosed embodiment is a package for a micromirror display device, often called a spatial light modulator, as is commonly used in projection displays. However, the package can be used for any MEMS device.
Micromirror projection display devices display an image by projecting light corresponding to the color channels of the image to be projected. A micromirror display device displays the pixels of the image by tilting mirror plates of micromirrors which, in one position, project light to the display (to display the pixel assigned to that mirror) and in the other position, deflect light away from the display (so as not to display the assigned pixel). It is important that the mirrors of micromirror devices tilt freely from one position to the other without any undesirable sticking at their end positions. To avoid sticking, the mirror packages usually contain an “antistiction” coating (usually referred to as “ASC”) which prevents such sticking.
The array of mirror plates in a micromirror device is commonly built on top of a CMOS wafer which contains all the required electrical circuitry for operation of the mirrors. In order for the device to operate reliably, all the mirrors and related structures need to be protected from ambient conditions by placing the device into a sealed, hermetic or semi-hermetic package. Such a package consists of three functional parts: (1) the micromirror substrate that includes the CMOS circuitry and the mirrors; (2) a transparent top cover (typically glass) which allows the incoming light beam to reach the mirrors so the light can be reflected towards the collecting optics; and (3) a seal which connects the micromirror substrate to the glass cover. The combination of these three parts creates a closed cavity surrounding the micromirror device structure.
The seal typically consists of epoxy which, during application, is in a viscous state so that it can easily be applied. After mating of the substrate and transparent cover, the epoxy must be cured into a hard material that provides the necessary mechanical strength to keep the package closed and sealed.
Curing epoxy, for example, using UV light or heat, will often result in the removal of solvents from the epoxy. Chemical analysis of these solvents shows that they contain benzene ring elements which are known to reduce the effect of the ASC used to keep the mirrors from sticking in one or the other of their end positions.
Modern fabrication techniques do not make micromirror devices one at a time. Instead, a wafer containing hundreds or more of the devices is manufactured and packaged all at once. The packaging process consists of the steps of: (1) dispensing the primary epoxy seal either on the substrate or on the glass cover; (2) mating the substrate and the glass cover; (3) curing the epoxy; and (4) dicing the wafer of sealed packages into individual, sealed and packaged devices (called “singulation”).
In order to maintain a defined tilt angle of the mirrors, it is desirable for a mirror to contact a landing post and be held against that post. When the mirror tilts over to the other side, the mirror will apply a substantial force to the landing post, which tends to displace the ASC on the post and to increase the adhesion of the mirror to the post. The displaced ASC needs to be replenished so as to recoat the surface of the landing post in order to minimize adhesion of the mirrors to the posts during the continuous operation of the mirrors. The ability to replenish the ASC depends, among other things, upon temperature, the presence or absence of moisture in the package and the quality of the ASC itself.
Therefore it is desirable to avoid ASC contamination. Such contamination can occur before and/or after introducing the ASC into the package. Experience has shown that epoxy outgassing of the package seal before the ASC is introduced can be a major source of ASC contamination. This contamination tends to increase the adhesion of the mirrors to the posts, both initially and later during reliability tests.
It is equally important not to allow moisture to get into the package. A high moisture level inside the package degrades device performance. Exposing the package to a combination of high temperature and humidity will cause moisture to get into the cavity containing the mirrors. A moisture-resistant seal is therefore required to avoid moisture from reaching the area of the package where the micromirror devices are.
One way of reducing moisture in the package is to reduce the seal thickness. As the seal material is the only material between the device substrate and the glass cover, the seal thickness must be thick enough, typically 10 um, to prevent the glass cover from ever touching the mirror surfaces at all angles to which the mirrors may be tilted during device operation. In addition, with less spacing between the substrate and the cover, defects in the glass surface will be closer to the mirror focal plane and hence more visible. Of course, as future devices become smaller, as they typically do, this minimum spacing will become smaller.
In order to reduce seal thickness to a more desirable thickness of less than 1 um, a spacer needs to be added on either the glass or the device side to prevent the mirrors from touching the glass cover and to render glass defects less visible. This spacer must be moisture-resistant. A highly moisture-resistant spacer can be achieved by bonding a third substrate to the glass cover, or by using Ni or Cu electroplating. But such solutions increase the number of process steps, complicating the manufacture of the devices and increasing their cost. As an alternative, a polymer resist material can be used to create a more cost effective physical structure, but these polymers have been found not to have the required moisture resistance required for micromirror devices.
One way the ASC can be applied to the packaged devices is through a hole in the seal, which afterwards must be closed, typically by using a plug. However, the moisture resistance around the plug tends to be less than that of the rest of the seal. Therefore creating a seal without using a plug is much more desirable. Furthermore, because plug-sealed devices must be individually tested after they sealed and diced, more manual handling is required, which can result in human error and is not well suited for mass production.
But creating a plugless seal requires that the ASC to be applied to the package before bonding of the two main parts, and the ASC, which is then present during sealing, must not be allowed to become degraded during the seal curing process. For example, creating a plugless seal using UV-curable epoxy in the presence of ASC beneath the seal material has been found to reduce the bonding strength of the seal.
Thermally cured epoxy results in acceptable seal bonding strength, even in the presence of ASC beneath the seal. However, the temperature required for curing thermal epoxy usually exceeds the ASC evaporation temperature, resulting in an undesirable loss of ASC during the sealing process.